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Author Topic: What triggers decoherence?  (Read 8670 times)

Offline Bill S

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What triggers decoherence?
« on: 13/05/2014 16:26:14 »
Decoherence

The Schrodinger’s Cat thought experiment is, unless you are a supporter of Eugene Wigner, generally solved by citing Decoherence.  The reasoning goes something like this: Instead of arguing that the intervention of a conscious observer causes the wave function of the quon to collapse; the interaction of the quon with its environment brings about Decoherence.  Typically, this involves interaction with other quons and brings about a situation similar to the older idea of a collapsing wave function.  No conscious observer is necessary for this, so the cat is not put into a state of superposition.  However, cats the world over cannot heave a collective sigh of relief, as Brian Clegg suggests they might (Dice World), because once in this box they will be either alive or dead at the end of the process.

Decoherence, it seems, can solve the cat problem, but does it raise another question of its own?  It would seem to.  Quons have been around for billions of years, presumably interacting with other quons in their environment.  Why, then, has decoherence not caused the wave function collapse of every quon in the Universe, long ago?

NB. Quon = quantum particle. http://en.wikipedia.org/wiki/Quon     


 

Offline JP

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Re: What triggers decoherence?
« Reply #1 on: 13/05/2014 19:04:06 »
I don't think anyone has a satisfactory answer to this, but I'm of the opinion that the universe is fundamentally quantum, so the cat state hasn't collapsed.  If the cat interacts with a quon, you just have two states: quon in state 1 + cat alive and quon in state 2 + cat dead (for example).  This is still a fully quantum system and can be in a superposition of two states.

If you push that further, it eliminates the observer problem to an extent because if you observe the cat but don't interact with me, I can treat you as being part of this quon-cat-Bill state which is in a superposition of alive and dead cats.  Once you tell me about what you see, I'm now a part of that state as well.

Of course, this just raises the question of why we never experience being in two states at once, which is probably a question about what consciousness means...
 

Offline Bill S

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Re: What triggers decoherence?
« Reply #2 on: 13/05/2014 21:20:32 »
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I don't think anyone has a satisfactory answer to this

Does that refer to “What triggers decoherence”, or “Why hasn’t every quon collapsed?

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so the cat state hasn't collapsed.
 

Does this mean that you see the cat as being in a superposition?

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If the cat interacts with a quon, you just have two states: quon in state 1 + cat alive and quon in state 2 + cat dead (for example).

If the cat interacts with the quon, would that not result in a dead or alive, rather than a dead and alive situation?

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…. if you observe the cat….

If I observe the cat, would that not bring about decoherence?

I suspect that I’m missing something fundamental here, so patience, please.
 

Offline JP

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Re: What triggers decoherence?
« Reply #3 on: 14/05/2014 01:03:00 »
If you observe the cat, you'd be in a superposition of "Bill sees cat alive" and "Bill sees cat dead."  Until I took a look at you to figure out which, you'd be in a quantum state.  When I looked at you, an observer looking at me would see me in a superposition state.

That's one way of looking at it, but it raises the question of consciousness again and why we won't ever observe ourselves as being in a superposition even if others might observe it.
 

Offline Bill S

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Re: What triggers decoherence?
« Reply #4 on: 14/05/2014 03:01:50 »
I think I must be further along the wrong track than I realised.  My understanding was that if I, or anyone else looked in the box the wave function would collapse, or decoherence would come about, and the cat would be dead or alive, whoever looked at it. 
 

Offline JP

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Re: What triggers decoherence?
« Reply #5 on: 14/05/2014 12:42:39 »
Part of the observer problem is this: we know if we have a cat in a superposition of alive and dead, we can hit it with a quon that will interact with it and take on one form if the cat is alive and another if its dead.  If this quon is not an "observer" we'd typically say the total state is now "cat+quon" and that the whole thing is in a superposition of "alive cat + quon state 1" + "dead cat + quon state 2" and that wouldn't collapse until we observer it.

But we don't know why, fundamentally, an "observer" should be any different than a quon interaction.  Fundamentally, we don't know if observation and interaction are different.  Is it correct to say that we collapse a wavefunction whereas the quon becomes part of the quantum state?  Or do we, like the quon, become part of the quantum state?  If the latter is true, we have to explain why we never experience being in a superposition, which likely requires explaining consciousness scientifically.  That's a whole can of worms.  So let's go back to the idea that we somehow collapse the wavefunction.

Now, take the cat + a single quon example above.  If we measure the quon, not the cat, we can still force the cat's state to collapse (since the cat and quon are now entangled in one state).  If you take literally (and most physicists do) that it is interaction, not conscious observation, that causes the wavefunction to collapse, we don't even have to be looking at the quon.  If we happen to bump into it, we can cause the state to collapse.  Now, replace quon with a region of the universe.  If we have contact with that region, even if we're not actively trying to measure anything, we can cause the cat to collapse, meaning that by the time we get near it, it has already assumed the alive or dead state.

If it's not clear, my own personal take is that we aren't special and that we become entangled with this whole quantum system.  I lean that way because science has continually demolished ideas based upon putting ourselves in a special place in the universe, so I suspect the same will be true here.  It is in no way a settled issue.
 

Offline cheryl j

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Re: What triggers decoherence?
« Reply #6 on: 14/05/2014 14:31:07 »
Or do we, like the quon, become part of the quantum state?  If the latter is true, we have to explain why we never experience being in a superposition, which likely requires explaining consciousness scientifically.

What are you expecting the experience of being in a superposition to be like? What could you notice about it that would be any different?
 

Offline JP

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Re: What triggers decoherence?
« Reply #7 on: 14/05/2014 16:10:11 »
Or do we, like the quon, become part of the quantum state?  If the latter is true, we have to explain why we never experience being in a superposition, which likely requires explaining consciousness scientifically.

What are you expecting the experience of being in a superposition to be like? What could you notice about it that would be any different?

I'd rather not speculate on that here, since it'd be veering off topic and into new theories.

What I will say is that as quantum mechanics stands now, if all we do is extend the idea of a quon to ourselves, we will end up in a superposition naturally.  We either have to make ourselves special ([it]e.g.[/it] consciousness somehow makes us non-quantum) or the laws of quantum mechanics have to break down as things get larger ([it]e.g.[/it] some have speculated that gravity may cause quantum mechanical effects to not extend to large scales).
 

Offline Bill S

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Re: What triggers decoherence?
« Reply #8 on: 14/05/2014 18:23:55 »
I suspect I may not have expressed my original question very well, so I’ll give it another shot.

1.  An unobserved quon is in an indeterminate state of superposition.  When it is observed, measured or interacts with anything in its environment it assumes definite characteristics.  This change is thermodynamically irreversible.

2.  If assertion 1 is correct, once a quon has decohered, there is no going back.  Photons may appear to be a special case, but I believe that is not so. 

3.  Decoherence can occur when one quon interacts with another.  No intelligent observation or measurement is needed.

4.  Given that 1-3 are correct; why have not all the quons in the Universe already succumbed to decoherence?   Have they?
     Are all quons that are in a state of quantum superposition artificially generated in experiments etc?
 

Offline JP

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Re: What triggers decoherence?
« Reply #9 on: 15/05/2014 02:36:14 »
I suspect I may not have expressed my original question very well, so I’ll give it another shot.

1.  An unobserved quon is in an indeterminate state of superposition.  When it is observed, measured or interacts with anything in its environment it assumes definite characteristics.  This change is thermodynamically irreversible.


There's a problem in the premise:

1. That is incorrect in the specific case that the "environment" consists of a second quon.  In this case, you now have 2 entangled quons in a superposition of 2 states.  This superposition is in essence a single quon (a single quantum state).
2. If the environment consists of N quons and we can model this as a bunch of individual quons, then we'll be in a superposition of N quons.

The real problem comes in that we know that wave collapse happens in the sense that we only observe a single state if we look at that quon.  So somehow we as observers aren't behaving like a quon.  The problem is that it is not obvious where to draw the line between observer that can cause collapse into a single state and interaction with another quon that just sets up a more complex superposition state.
 

Offline Bill S

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Re: What triggers decoherence?
« Reply #10 on: 15/05/2014 15:07:43 »
Thanks JP.  I think I’m getting there slowly.  My trouble is that I have difficulty letting go of something until I feel I understand it; and I know that QM does not lend itself to understanding, especially by lay people.


 

Offline JP

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Re: What triggers decoherence?
« Reply #11 on: 15/05/2014 16:04:28 »
True.  It is probably best to build QM from the ground up if you want to understand it rather than jumping in to the really interesting, but cutting-edge and complex ideas like decoherence. 

I would say the starting point is to just consider the interaction of 2 quons.  Let's say quon 1 starts in a superposition of two states, 1A and 1B (the 1 signifies it is quon 1 and the A/B signifies one of two possible states). 

Now quon 1 interacts with quon 2.  In reality, interactions can be complicated, but in our thought experiment, let's say they interact in such a way that quon 2 goes into the same A or B state as quon 1.  To describe this, we now have a state that is a superposition of 2 states: 1A+2A and 1B+2B.  This is entanglement, by the way--if I measure particle 1 to be in a state, I know that particle 2 is in a matching state.

Assuming all particles behave like these quons, after interaction with N quons, we'll have a superposition of 2 states:
1A+2A+3A+4A+...+NA and 1B+2B+3B+4B+...NB
This is still fundamentally 2 quantum states in a superposition.

Before we go further, does that make sense?  We can discuss observers and wavefunction collapse next, but rather than make a long post, I figure the best way is to take one fundamental point at a time.
 

Offline Bill S

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Re: What triggers decoherence?
« Reply #12 on: 15/05/2014 23:46:20 »
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This is still fundamentally 2 quantum states in a superposition.

This is because there are only two states, irrespective of how many quons are involved?
 

Offline JP

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Re: What triggers decoherence?
« Reply #13 on: 16/05/2014 01:31:34 »
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This is still fundamentally 2 quantum states in a superposition.

This is because there are only two states, irrespective of how many quons are involved?

No, it's because I chose a simple example.  :p

In general, you can get lots of states depending on the states of the 2 quons and how they interact.
 

Offline cheryl j

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Re: What triggers decoherence?
« Reply #14 on: 16/05/2014 07:58:56 »
4.  Given that 1-3 are correct; why have not all the quons in the Universe already succumbed to decoherence?   Have they?
     Are all quons that are in a state of quantum superposition artificially generated in experiments etc?


Wouldn't new quantum events just continue to happen? I'm not sure I understand why they would become all used up or never happen again because of decoherence. Would it be theoretically impossible to fire the same particle next week through the double slit?

 

Offline Bill S

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Re: What triggers decoherence?
« Reply #15 on: 16/05/2014 20:04:49 »
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No, it's because I chose a simple example.  :p

My question referred to your simple example, I do admit it was a bit naive. :)

I'm going to try to find time over the next few days to put together the information from this thread, and a few other places, to see how much sense I can make of it.  In the words of the famous quote: "I'll be back."
 

Offline Bill S

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Re: What triggers decoherence?
« Reply #16 on: 16/05/2014 23:28:10 »
So much for that plan of action!


JP, I think I’m not going to get very far with this unless/until I sort out some bits from your early posts.  You say:

“If the cat interacts with a quon, you just have two states: quon in state 1 + cat alive and quon in state 2 + cat dead (for example).  This is still a fully quantum system and can be in a superposition of two states.”

Just to be clear; you are saying that the cat is in a superposition of alive and dead before it is observed?   If so, is this the generally accepted interpretation?

 

Offline evan_au

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Re: What triggers decoherence?
« Reply #17 on: 16/05/2014 23:28:31 »
In my simplistic view:
  • On a macroscopic scale, as soon as the cat sees its own tail, the cat will take on the state of "cat is alive".
  • On a microscopic scale, as soon as a blood cell bumps into the wall of a blood vessel, the cat will be alive.
  • On a quantum scale, as soon as a photon bounces off the cat, the cat will take on some state (dead or alive, assuming the transition from one state to the other is infinitely fast...). 
Interactions which do not cause the quantum state to decohere are the exception. Particles which interact and end up with the extra particle entangled with the first in a controlled manner are an exception, especially in a human-friendly environment around 300C. That's what makes quantum computers so difficult to develop, even near Absolute Zero.
 

Offline Bill S

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Re: What triggers decoherence?
« Reply #18 on: 18/05/2014 19:30:43 »
I tried pulling the threads together with some apparent success, but I find I am wanting to ask questions that are based on a probable lack of secure foundations; so, JP, I’m coming back to your suggestion of foundation building, if that's still OK with you.

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I would say the starting point is to just consider the interaction of 2 quons.  Let's say quon 1 starts in a superposition of two states, 1A and 1B (the 1 signifies it is quon 1 and the A/B signifies one of two possible states).

I’m OK with that. Step one towards becoming an expert. :D

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Now quon 1 interacts with quon 2.  In reality, interactions can be complicated, but in our thought experiment, let's say they interact in such a way that quon 2 goes into the same A or B state as quon 1.  To describe this, we now have a state that is a superposition of 2 states: 1A+2A and 1B+2B.  This is entanglement, by the way--if I measure particle 1 to be in a state, I know that particle 2 is in a matching state.

Discounting possible complications, I’m with this.  I’m not going to ask why this interaction might happen, rather than another, because I guess that would come later.

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  Assuming all particles behave like these quons, after interaction with N quons, we'll have a superposition of 2 states:1A+2A+3A+4A+...+NA and 1B+2B+3B+4B+...NB

I understand this as saying that after any number of identical interactions, we would have two quantum states, with all the involved particles in a quantum superposition of both states.  Is that right? 

 
 

Offline JP

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Re: What triggers decoherence?
« Reply #19 on: 18/05/2014 20:48:56 »
No problem, Bill.  I think too often threads in this forum try to jump into the deep end of the pool without building foundations.  Some topics are virtually impossible to really come to grips with without foundations and I think this is one of them. 

On your questions:
1) I know you didn't ask about why this particular interaction happens, but the idea is that the interaction here is just a simple thought experiment, so we chose an idealized interaction that's simple to understand. 

2) Yes, after any number of interactions, we still have 2 states which are built up from the original 2 states.  So even though the particle has interacted with many other particles, it is still in a quantum superposition of two states. 

The point of this is to show that it is not simply "interacting with a lot of other particles" that causes collapse. 
 

Offline Bill S

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Re: What triggers decoherence?
« Reply #20 on: 19/05/2014 17:25:18 »
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The point of this is to show that it is not simply "interacting with a lot of other particles" that causes collapse.

So far, so good.  What would you recommend as the next step?
 

Offline JP

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Re: What triggers decoherence?
« Reply #21 on: 19/05/2014 19:26:54 »
There's a couple ways to go.  In one sense, we've answered the question in this thread of whether decoherence explains wavefunction collapse (it doesn't).  How so? 

We have to make 1 big assumption: every interaction is fundamentally quantum in nature.  This means that these interactions are also unitary, which is a technical term that implies that information is preserved in this interaction.  I.e. if the initial quon is in superposition and it interacts with a second quon, the system consisting of both quons contains that superposition information.  It may not be as simple as the case above, because the second quon will likely bring in its own superposition, but we haven't lost the superposition, i.e. the quantum-ness of the first quon. 

Once we assume this, then every interaction no matter how complex, can be broken down into these information-preserving quantum interactions.  So if we draw a big box around everything our first quon has interacted with, we haven't lost information about the quantum-ness of it and can in theory recover its superposition. 

What decoherence says is that to get full information about the quantum state of my quon, I need to have full information about the current quon and everything it's interacted with.  Typically, this is impossible for all but very isolated systems just due to the number of particles you'd need to keep track of.  So in practice, even if we can characterize a portion of the system very well, we can't fully recover information about our initial quon: some portion of that information is tied up with particles that we simply don't have the technical ability to measure.  The important point is that in theory the information is still there and nothing has collapsed.  It's just that in practice we'll never be able to design a system good enough to capture this information. 
 

Offline Bill S

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Re: What triggers decoherence?
« Reply #22 on: 19/05/2014 22:19:50 »
 
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if the initial quon is in superposition and it interacts with a second quon
If the initial quon is not in superposition, would this mean that the interaction was not “quantum in nature”, or would the fact of the second quon being in superposition ensure the quantum nature of the reaction? 

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.  So if we draw a big box around everything our first quon has interacted with, we haven't lost information about the quantum-ness of it……

Presumably this applies to the second quon as well.

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…… and can in theory recover its superposition.

Just checking on the word “recover”.   I assume it is “we” who do the recovering, as in learning about; not that the quon had lost its quantum-ness, and needs to get it back.

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What decoherence says is that to get full information about the quantum state of my quon, I need to have full information about the current quon and everything it's interacted with.

This is where I may go off the rails.  What I am getting from this is that decoherence does not come about as a result of quantum reactions between quons, but is a function of the extraction of information from a quantum system.
 

Offline JP

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Re: What triggers decoherence?
« Reply #23 on: 20/05/2014 03:54:03 »
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if the initial quon is in superposition and it interacts with a second quon
If the initial quon is not in superposition, would this mean that the interaction was not “quantum in nature”, or would the fact of the second quon being in superposition ensure the quantum nature of the reaction? 
This requires a bit of quantum mechanics to nail in detail, but every particle is in a superposition if you measure the right variable.  What I am assuming is that quantum mechanics underlies the behavior of quons.

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Quote
.  So if we draw a big box around everything our first quon has interacted with, we haven't lost information about the quantum-ness of it……

Presumably this applies to the second quon as well.
Yes, basically whatever quantum information goes in has to come out.  You can't destroy information in quantum processes.

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…… and can in theory recover its superposition.

Just checking on the word “recover”.   I assume it is “we” who do the recovering, as in learning about; not that the quon had lost its quantum-ness, and needs to get it back.
A better statement would be that all the information put into the box is still in there.  Nothing has been irrevocably lost.  In a handwaving way, although your original quon might be a mess once it's interacted with all the other quons in the box, information about its initial state is still preserved in there somehow.

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What decoherence says is that to get full information about the quantum state of my quon, I need to have full information about the current quon and everything it's interacted with.

This is where I may go off the rails.  What I am getting from this is that decoherence does not come about as a result of quantum reactions between quons, but is a function of the extraction of information from a quantum system.

Decoherence is based on a quantum version of statistical mechanics.  Although information about my first quon is in there, it may be jumbled up with all the other quons in such a way that I can't recover it unless I get the information from all the quons. 

This isn't all that far off from classical statistical mechanics.  If I have a box of a billion classical particles (think little billiard balls rolling about rapidly) and I add to that box a single billiard ball with unknown velocity and I let them bounce around for a while, can I figure out the unknown velocity by looking in the box?  Technically, yes, if I measure all the particles.  But practically, no, since it is impractical to try to do that.  Our initial billiard ball may still be identifiable and has all the properties: mass, velocity, position, of a billiard ball, but we can't figure out its initial state unless we measure all billion other billiard balls it's interacted with.

Quantum mechanically, it gets a bit more complicated, but its a similar idea.  Our initial quon is still quantum mechanical.  It still can behave like a wave or a particle and have all those nice, confusing properties of a quon.  But to get information out about its initial state back out, we need to know detailed information about everything its interacted with and this is practically impossible in many cases.
 
One place decoherence is very useful is in explaining why quantum computers are so hard to get working (they've been '10 years off' for several decades.)  If we put a qubit in to be stored, we want to be able to maintain information about its quantum state.  But if it interacts with anything else, that information gets tangled up with the other particles it has interacted with.  Very quickly, unless it's held in extremely careful isolation, we lose useful information about its state to the environment and the environment leaks useless (to us) information into the qubit.  It doesn't collapse--it's still a quantum particle--but it has been corrupted by the environment.  If we really wanted to use that information, we'd have to look at everything it's interacted with to back out the information.  Things get even harder when you have to move qubits around and get them to interact to do useful processing with them. 
 

Offline Bill S

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Re: What triggers decoherence?
« Reply #24 on: 20/05/2014 21:30:16 »
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What I am assuming is that quantum mechanics underlies the behavior of quons.

I imagine it would be difficult to do anything in QM without this assumption.

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although your original quon might be a mess once it's interacted with all the other quons in the box, information about its initial state is still preserved in there somehow.

Would this reflect the difference between waveform collapse and decoherence? 

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Decoherence is based on a quantum version of statistical mechanics........
 

I know I've said this before, but if you have not already written a pop sci book, you should; and if you have, I want it!

The reason I included the “Schrödingcat” in the OP is that most of the pop sci explanations I have seen have been attempts to describe why the cat would not be in superposition.  How far off track would it be to ask if the cat might not be in superposition, but the individual quons of which the cat is composed might be?
 

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Re: What triggers decoherence?
« Reply #24 on: 20/05/2014 21:30:16 »

 

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